This invention relates to automatic equalizers for compensating distorting data transmission channels and more particularly to rapid initial adjustment of such equalizers with respect to channel characteristics.
Intersymbol interference due to the overlapping of response components of consecutive signals is a serious impairment in synchronous digital data transmission over voiceband telephone channels. Some kind of automatic equalization is therefore necessary when transmitting high-speed data over such a channel with unknown characteristics. The equalizer generally consists of a transversal filter with adjustable tap coefficients.
If the response of the transmission channel to a single pulse and its noise characteristics are known to the receiver, the optimum tap gains can theoretically be calculated from a system of simultaneous equations. Automatic equalizers solve these equations by iterative algorithms, which lead to results of sufficient precision for practical use after a finite number of iterations. After initial adjustment, the receiver associated with the equalizer is ready for data reception.
In an increasing number of today's applications, high-speed data messages are transmitted in short bursts. Such applications occur in polling situations including airline reservation, inventory control and banking systems. Data set start-up time seriously limits the efficiency of such systems when it approaches or exceeds the actual message time.
The efficiency of a data transmission system relative to start-up time (neglecting roundtrip delays) may be defined as EQU .epsilon.=T.sub.M /(T.sub.M +T.sub.S),
where T.sub.M and T.sub.S are respectively message time and start-up time. A typical polling message of 120-bit length can be transmitted in time T.sub.M =12 miliseconds at a 9,600-bit-per-second rate. However, a representative data set (not arranged for fast start up) capable of this transmission rate needs the time T.sub.S =250 miliseconds to start up. High-speed data sets themselves often require longer absolute start-up times because more sophisticated carrier and timing control as well as equalizer control are required. In this example 95 percents of the connection time for the message is needed for start up, while only 5 percents is used for actual information transfer.
In U.S. Pat. No. 3,715,666, an equalizer is proposed in which the start-up time is shortened. However, in the proposed equalizer great reduction of the start-up time cannot be obtained since the tap coefficients are sequentially determined.